<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            margin: 0;
        }
        .box{
            width: 100px;
            height: 100px;
            background-color: #f00;
            position: absolute;
            /* left right top bottom */
            /* right:100px;
            bottom:100px; */
            top:0;
        }
    </style>
</head>
<body>
    <!-- 
        文档流
        定位：position
            绝对定位 absolute ,针对**文档**定位，脱离文档流,提升了层级
    -->
    123
    <div class="box"></div>
    456
</body>
</html>